In rolling out a conventional wireless carrier network, one of the primary considerations is the process of selecting and allocating frequency channels for all of the cellular base stations within the system. This process, which is called frequency reuse or frequency planning, depends on various factors such as frequencies available for use, cell geometry, type of antenna, and topography.
A key parameter in determining frequency reuse is the Carrier-to-Interference (C/I) ratio, which measures the ratio of the power level of the radio frequency carrier to the power level of the interference signal in the channel. The C/I ratio helps to determine the maximum interference level that will still allow a cellular system configuration to provide an acceptable quality of service.
In rolling out a new GSM outdoor macro base station network, assuming a standard 4/12 geometry cell cluster reuse pattern (see FIG. 1), a minimum of 12 frequencies is typically required to keep the quality of service within tolerable limits. For the GSM network, that means meeting or exceeding the GSM 9 db C/I ratio specification.
In rolling out a new GSM outdoor micro or pico base station network, one of several frequency planning strategies may be implemented. One strategy is to allocate new (unused) frequencies to the micro/pico cells, depending on the availability of unused frequencies in the carrier's inventory. Alternatively, the carrier may choose to share the same frequencies allocated to the existing macro cell network. In either case, assuming a standard 4/12 geometric cell cluster reuse pattern, a minimum of 9 to 12 frequencies is typically required to meet or exceed the GSM 9 db C/I quality of service specification for the micro/pico cell network. The reason for the reduced number of frequencies is that the micro or pico cells are deployed below the clutter height, which means a higher signal loss to more distant areas, effectively reducing the interference level.
When considering the rollout of yet another network of base stations, specifically indoor pico or personal base stations, the superior frequency planning strategy would be to allocate new (unused) frequencies, in order to avoid interference from the more powerful outdoor macro stations, particularly in high rise structures. Although allocating new (unused) frequencies is a superior strategy (easier to implement) than sharing the frequencies with the macro- and micro cells when rolling out a new indoor network, it is not always feasible, for several reasons.
First, most carriers do not own enough additional frequencies to implement the unused frequency strategy. Typically, the only unused frequencies in a carrier bandwidth inventory are the two “guard” frequencies on the extreme ends of the carrier's licensed bandwidth. These frequencies, however, are typically unusable in a practical sense because of potential interference from frequencies licensed and deployed by other carriers. Secondly, even if these two guard frequencies were used, they would not allow the carrier to meet or exceed the current GSM 9 db C/I ratio quality of service specification discussed above.
From the carriers point of view, an ideal solution to their frequency planning problem would be a method or mechanism allowing the rollout of a GSM network of indoor pico or personal base stations that fulfills the following criteria: a) use of only one or two unused frequencies, preferably guard frequencies, b) satisfaction of the GSM 9 db C/I ratio qualify of service specification, and c) seamless integration with the carriers existing outdoor macro/micro network.
Conventional Timeslot Allocation Management: Conventional networks use time slot allocation management to help control mobile station interference within a single cell, rather than between cells. A base station or base station controller allocates time slots within channels to all of the mobile stations within its cell, ensuring that no two mobiles are transmitting or receiving signals within the same time slots, thereby avoiding any interference between mobile stations within a particular cell. In addition, the mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed or a handover should be initiated.
Conventional Channel Structure and Use of Timeslots: Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time and Frequency Division Multiple Access (TDMA/FDMA). The FDMA part involves division by frequency of the total MHz bandwidth into allocatable carrier frequencies of 200 kHz bandwidth. One or more carrier frequencies are then assigned to each base station. Each carrier frequency consists of 2 200 kHz channels separated by a duplex distance (e.g. 45 MHz in GSM 900). One frequency is used for the downlink (BTS→MS) and the other frequency is used for the uplink (MS→BTS). The pair of one 200 kHz channels is called a duplex channel.
Each of these duplex channels is then divided in time, using a TDMA scheme, into eight time slots. Groups of eight consecutive time slots form TDMA frames, each with duration of 4.615 ms. Each time slot is a burst period (BP) during which a transmission burst of modulated bits is broadcast. One time slot is used for transmission by the mobile (uplink) and one for reception (downlink). They are separated in time so that the mobile unit does not receive and transmit at the same time, a fact that simplifies the electronics.
The GSM BP lasts 15/26 milliseconds (ms) (or approximately 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approximately 4.615 ms), which forms the basic unit for the definition of logical channels, an endlessly recurring cycle of BP time slot transmissions.
Logical channels are defined by the number and position of their corresponding burst periods or time slots. The logical channels are used to exchange information between mobile stations and base stations. The logical channels are divided into dedicated channels, which are allocated to a mobile station, and common channels, which are used by mobile stations in idle mode. Within a logical channel, the transmission (downlink) to a mobile station occurs 3 timeslots earlier than the reception (uplink) from a mobile station.
The first carrier within a cell is called the Broadcast Control Channel (BCCH) carrier. The BCCH carrier transmits BCCH system information over timeslot 0, plus Access Grant Channels, Paging channels and most often SDCCH channels. The BCCH carrier has to be on at all times, so the mobiles in surrounding cells and in its cell can check the BCCH carrier signal on all timeslots. Another characteristic of the BCCH carrier signal is the base station transmitting the BCCH carrier signal does so with a constant output power. Even if traffic channels are in active use, creating potential interference with the BCCH carrier signal, the BCCH carrier signal is still transmitted with a constant output power on all timeslots. All other frequency carriers of a cell (TCH carriers) can be switched of if there is no traffic on the carrier/timeslot.
Conventional Power Control: To minimize co-channel interference and conserve power, both the mobiles and the base transceiver stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 milliwatts) or 2.5 mW in GSM 1900. The power control is typically done on the TCH carriers. Mobile and base stations need only transmit enough power to make a connection. Any more is superfluous, and using less power means less interference.
The mobile and base station measures the signal strength and signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed in either the mobile or the base station. Power control needs to be handled carefully, since there is the possibility of instability. This arises from having a mobile increase its power in response to increased co-channel interference caused by another mobile increasing its power.
In contrast to conventional use of GSM timeslot allocation management to control interference, the present invention uses a timeslot allocation management to reduce the number of frequencies required to control interference between neighboring cells (intercell interference control). A mechanism for such a capability is provided for both macro base stations and pico or personal base stations.